It is well-known that friction materials which are intended for use in heavy duty brake or clutch operations in motor vehicles must withstand severe service conditions. In use they are subjected to rigorous treatment by repeated and prolonged braking or clutching applications which develop high temperatures, usually about 500.degree. F., in the friction materials, these temperatures sometimes exceeding 1000.degree. F. on the friction surface of the material and progressively decreasing inwardly of such surface. These high temperatures, especially when occurring during high speed stops or following repeated applications of the brakes or clutches, tend to decompose the organic material which acts as a binder in the friction material. Such decomposition results in the formation of gaseous or liquid products which in some cases cause loss of braking or clutching efficiency. Moreover, the weakened decomposed lining surface may wear faster, with resultant premature failure of these surfaces to perform their intended function.
In general, a friction material contains a matrix or binder, such as a thermosetting resin, a high-melting fibrous material, and various additives which contribute friction and/or wear properties and bulk. In the past the fibrous material has invariably been chrysotile asbestos. Asbestos fiber imparts strength to the friction material and, at the same time, has a beneficial effect on friction properties. Moreover, in the past, it has been relatively cheap.
However, asbestos deposits throughout the world are rapidly being depleted, with the result that its cost is increasing. Moreover, the continued use of asbestos has recently come under a cloud because of the realization that it presents a serious cancer hazard for those who work with it. There has been an increasing concern about such hazard with the result that the expense of health precautions now required for the safe handling of asbestos has become a significant burden. Accordingly, it has become desirable to eliminate asbestos from frictional material formulations and to find a replacement.
U.S. Pat. No. 2,394,783 (Keller et al.) shows a friction element containing a butadiene-acrylonitrile copolymer as a bonding agent. The two examples in this patent show such elements which also contain asbestos, zinc oxide and a phenol-formaldehyde resin. These compositions are said to be especially useful because they withstand severe usage and high temperatures without undergoing excessive thermal degradation and loss of friction qualities.
U.S. Pat. No. 3,007,890 (Twiss et al.) shows a heat-cured friction material containing a butadiene-acrylonitrile copolymer, less than 15% of a phenolformaldehyde resin and asbestos. Although asbestos is preferred, wollastonite is included in a list of "filamentous, particulate materials which may be used in its place." The friction materials may also contain zinc oxide and molybdenum sulfide.
U.S. Pat. No. 3,494,884 (Kraft) deals with a friction material which contains an intimate mixture of granules of a sintered metallic friction material, a filler and an organic binder. The sintered metallic material may include molybdenum disulfide; the filler may be glass wool; and the organic binder may be a phenol-formaldehyde resin.
U.S. Pat. No.3,725,334 (Popp et al.) teaches the stabilization of a phenolic resin by means of a particularly defined cross-linked polyisocyanurate. These stabilized resins are said to be useful in friction materials and a friction material is shown containing such a resin, zinc oxide, asbestos, molybdenum sulfide and an acrylonitrile-butadiene copolymer.
U.S. Pat. No. 3,832,325 (Eschen) relates to composition brake blocks comprising SBR or nitrile rubber plus a small proportion of zinc oxide, asbestos fiber, cast iron chips, a lead component, a thermosetting resin and sulfur.
U.K. Patent Application No.. 2,003,088 shows a disc brake pad the friction material of which may comprise a wide variety of ingredients including glass fibers, phenol-formaldehyde, an acrylonitrile rubber, zinc oxide and molybdenum sulfide.